1. Field of the Invention
The present invention relates to an optical pickup apparatus that performs an operation of reading signals recorded in an optical disc and an operation of recording signals into an optical disc with a laser beam.
2. Description of the Related Art
Optical disc apparatuses are widely used that can apply a laser beam emitted from an optical pickup apparatus to a signal recording layer of an optical disc to perform a signal reproducing operation and a signal recording operation.
Although optical disc apparatuses using optical discs called CD and DVD are generally used, optical disc apparatuses are recently commercialized that use optical discs with record densities improved, i.e., optical discs of the Blu-ray standard.
A laser beam having a shorter wavelength, e.g., blue-violet light having a wavelength of 405 nm is used as a laser beam for performing an operation of reading signals recorded in a Blu-ray standard optical disc. A protective layer provided on a top surface of a signal recording layer of the Blu-ray standard optical disc has a thickness of 0.1 mm, and a numerical aperture of an objective lens is specified at 0.85 which is used for performing the operation of reading signals from the signal recording layer.
The optical pickup apparatus is configured so as to be capable of controlling a drive current that is supplied to a laser diode to acquire a laser output suitable for reading signals recorded in an optical disc or to acquire a laser output suitable for recording signals into an optical disc.
The optical pickup apparatus is configured so as to be capable of performing a control operation for focusing a spot, which is formed through an operation of condensing a laser beam emitted from a laser diode by an objective lens, onto a signal recording layer of an optical disc, i.e., a focusing control operation, and a control operation for causing a spot of a laser beam to follow a signal track, i.e., a tracking control operation.
The optical pickup apparatus has such a problem that an aberration called a spherical aberration is caused due to a thickness of a protective layer located between a surface of an optical disc, i.e., an incident surface of a laser beam and a signal recording and an operation of reproducing signals and/or an operation of recording signals is not normally performed. Thus, an art for solving such a problem has been developed.
Among recent optical discs, a two-layer optical disc having two signal recording layers is commercialized as a product in order to increase a signal recording capacity. Although an operation of moving an objective lens in a light axis direction is performed in order to read signals recorded on the signal recording layers included in such a two-layer optical disc, a large spherical aberration is caused when an operation of jumping to a different signal recording layer is performed, since a transparent protective layer, which is provided between each of the signal recording layers and the incident surface of the laser beam, significantly changes in thickness.
Such an art has been developed that the spherical aberration, caused due to the operation of jumping between the signal recording layers, is corrected through an operation of moving a collimating lens in the light axis direction.
Some optical pickup apparatuses are configured such that operations of reading signals recorded in optical discs of two standards having protective layers different in thicknesses are performed by using an objective lens provided with a plurality of focal points (see Japanese Laid-Open Patent Publication Nos. 2006-147069, 2009-199676, and 8-315402).
The optical pickup apparatus described in Japanese Laid-Open Patent Publication No. 2009-199676 is configured such that an operation of correcting the spherical aberration, which is caused due to change in thickness of the protective layer, is performed by moving the collimating lens in the light axis direction, when an operation of moving a position of light condensed by the objective lens, i.e., a position of a focal point, is executed in order to perform an operation of reading signals recorded on one signal recording layer after performing an operation of reading signals recorded on the other signal recording layer.
Description will then be given of the operation of correcting the spherical aberration when a conventional optical pickup apparatus is used for reading signals recorded on signal recording layers of a two-layer optical disc specified by the Blu-ray standard.
In the two-layer optical disc of the Blu-ray standard, specifications are such that 0.075 mm is a distance between the surface and a first signal recording layer provided in a location closer to the surface of the optical disc, and 0.1 mm is a distance between the surface and a second signal recording layer provided in a location further from the surface of the optical disc.
As apparent from such specifications, is 0.025 mm is a distance between the first signal recording layer and the second signal recording layer, and a spherical aberration is caused based on such a difference in thickness of the protective layer. The operation of moving the collimating lens in the light axis direction is performed in the optical pickup apparatus in order to correct such a spherical aberration.
When a thickness of the protective layer is changed by 0.001 mm in such an optical disc, a spherical aberration of 10 mλrms is caused. Since a thickness of the protective layer is 0.025 mm between the first signal recording layer and the second signal recording layer, if a laser-beam condensed position is moved from the first signal recording layer to the second signal recording layer or from the second signal recording layer to the first signal recording layer by the operation of moving the objective lens in the light axis direction, a spherical aberration of 10 mλrms×25=250 mλrms is caused.
In an optical pickup apparatus configured such that signals recorded on the first signal recording layer and the second signal recording layer included in a two-layer optical disc are read with a spot formed by the condensing operation of a single-focus objective lens, a distance that the objective lens is moved is substantially equal to a distance between the layers. Since an amount of a spherical aberration changes in accordance with the movement distance of the objective lens and changes in proportion to a thickness of a protective layer, an amount of the spherical aberration increases as the movement distance of the objective lens increases.
On the other hand, in an optical pickup apparatus configured such that a spherical aberration of 18 mλrms can be corrected when the collimating lens is moved by 0.1 mm in the light axis direction, the collimating lens is required to be moved by 0.1×250/18=1.39 mm in the light axis direction in order to perform the operation of correcting the spherical aberration of 250 mλrms, and therefore, an amount of moving the collimating lens becomes greater.
If such an amount of moving the collimating lens becomes greater, not only the optical pickup apparatus becomes greater in shape but also a time for moving the collimating lens becomes longer, and therefore such a problem arises that a longer time is required for achieving a state where the operation of reading signals recorded on the signal recording layers is performed in the jump operation.